Spread spectrum communications is being used for a number of commercial applications and is proliferating at a rapid rate. Orthogonal code division multiple access (OCDMA) has been proposed (see U.S. Pat. No. 5,375,140 "WIRELESS DIRECT SEQUENCE SPREAD SPECTRUM DIGITAL CELLULAR TELEPHONE SYSTEM", and U.S. Ser. No. 08/257,324, filed Jun. 7, 1994, incorporated herein by reference) as an effective technique for improving the capacity, i.e., bandwidth efficiency, of the more conventional quasi-orthogonal CDMA.
In conventional direct sequence (DS) spread spectrum CDMA systems, the individual users transmit on the same frequency using different pseudo-noise (PN) codes. The PN codes are quasi-orthogonal, i.e. they have relatively low but nonzero cross-correlation values with each other.
In an OCDMA system, each user is assigned a code which is orthogonal to all of the other user codes (i.e. the orthogonal codes have a cross-correlation value of zero with each other). Further, the orthogonal code period is chosen such that the code repeats an integer number of times (usually once) in data symbol time. The code epoch is synchronized with the symbol transitions so that no data transitions occur within the code.
The number of users is limited by the number of orthogonal functions available, which for binary codes is equal, at most, to the length of the code. An example is the set of Radamacher-Walsh functions for which there are 2" orthogonal functions of length 2" where n is a positive integer. Note that the chipping rate is equal to the maximum number of orthogonal users times the symbol rate. This implies that a high data rate requires a much higher chipping rate.
OCDMA systems are designed such that all signals are received in time and frequency synchronism. Thus all users remain orthogonal to each other and,in an ideal world, any user can be recovered with no multiple access noise from other users. This is most practical in a star configured network where a multiplicity of users transmit to and receive from a single hub station. This configuration is often used in satellite networks.
There are, of course, a number of practical considerations and real-world effects that cause OCDMA performance to degrade from ideal. For example, multipath returns that are delayed a significant portion of a chip are no longer truly orthogonal and cause access noise. This is a problem for high data rate systems, since the chipping rate is correspondingly higher, and the multipath delay spread becomes increasingly significant. A technique for combating this effect is disclosed in Magill U.S. patent application Ser. No. 08/352,313, filed Dec. 8, 1994 entitled "ORTHOGONAL CODE DIVISION MULTIPLE ACCESS COMMUNICATION SYSTEM HAVING MULTICARRIER MODULATION", also incorporated herein by reference. In this application, it is disclosed that multiple OCDMA signals be transmitted on orthogonally spaced carriers (i.e. spaced at the chipping rate) and the data from a single user is demultiplexed onto the multiple carriers. In this way, the chipping rate is reduced by the number of carriers.